The present invention relates to a data storage device and a control method therefor, and more particularly to a data storage device which determines whether the flying height of each head is normal, and a control method therefor.
Known data storage devices use various types of media such as optical disks and magnetic tapes. Among these data storage devices are hard disk drives (HDDs), which have been widely used as storage for computers and are now essential storage devices in computer systems. Their applications, however, are not limited to computers. Due to their superior characteristics, HDDs have found application in an increasing number of fields. For example, they have been used in video recording/reproducing apparatuses and recording/reproducing apparatuses for car navigation systems and also used as removable storage (memory) for digital cameras.
A HDD comprises: at least one magnetic disk for storing data; at least one head device portion for writing/reading data to/from the magnetic disk; at least one slider to which the head device is fixed; and an actuator for holding the slider and moving the head device portion to a desired position over the magnetic disk. A voice coil motor rotates the actuator around a pivot so as to move the head in a radial direction over the rotating magnetic disk. This allows the head device portion to access a desired track formed on the magnetic disk to read/write data.
The actuator has a resilient suspension to which the slider is fixed. The pressure (to the head device portion) generated due to the viscosity of the air between the rotating magnetic disk and the ABS (Air Bearing Surface) surface of the slider facing the magnetic disk balances with the pressure applied (to the head device portion) by the actuator toward the magnetic disk, causing the head device portion to float above the magnetic disk with a certain gap therebetween. Such a gap between the head device portion and the magnetic disk is referred to as a head flying height.
In some cases, the head flying height becomes abnormally high while the HDD is operating. This phenomenon is considered to be attributed to thermal asperity on the magnetic disk, a disturbance in the air bearing due to dust, or an external shock. If the head device portion floats abnormally high, the level of the magnetic signal applied from the head device portion to the magnetic disk decreases. If a write operation is performed at that time, the data may be properly written, or no data may be written at all.
To prevent occurrence of such an error in a write operation, a technique has been proposed which detects the head flying height based on the servo gain and inhibits the data write operation when the flying height is abnormal (see Japanese Patent Laid-Open No. 2001-229637). In the HDD, the read signal read out from the magnetic disk is amplified to a certain potential through AGC (Automatic Gain Control). The level of the read signal read by the head device portion is inversely proportional to the head flying height. Therefore, the higher the flying height, the larger the gain of the AGC. The technique disclosed in Japanese Patent Laid-Open No. 2001-229637 monitors the gain of the AGC for amplification of the servo signal at the start of a write operation and compares its value with a reference value to detect an abnormality in the head flying height. If the flying height is determined to be abnormal, the technique inhibits the data write operation.
Japanese Patent Laid-Open No. Hei 09-139040 (1997) discloses a system for monitoring changes in the head flying height based on the AGC voltage of the AGC circuit of the data reproduction processing circuit which receives the reproduced signal obtained when data stored on a disk is read out by the head and which performs predetermined data reproduction processing. The CPU compares the input AGC voltage with threshold value data DT stored in memory, and if the voltage falls outside a permissible range, the CPU outputs a flying height change detection signal FS indicating occurrence of an abnormal change in the head flying height. Further, the system disclosed measures the AGC voltage CV corresponding to the reproduced signal from the head and compares an average value CVa of the AGC voltage CV with the threshold value data DT to determine whether the AGC voltage CV falls within a permissible range indicated by the threshold value data DT.